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Geology is the Way

Olivine

Orthorombic

(Mg, Fe)2[SiO4]

Olivine is an important mafic orthosilicate occurring in basaltic and gabbroic rocks in the Earth’s crust. It is also the most abundant mineral in the Earth’s mantle, constituting more than 50% of the upper mantle. The name ‘olivine’ derives from its distinctive olive/bottle green to yellowish color.

Structure and chemistry
Members of the olivine group are orthosilicates with isolated SiO4 tetrahedrons surrounded by octahedral M-sites that contain (Mg,Fe)2+ cations, surrounded by six oxygens. The SiO4 tetrahedrons alternate in the structure, pointing alternatively up and down perpendicularly to the c-axis. The M-sites include a regular set of octahedral sites (M1) and a series of distorted octahedral sites (M2). Each oxygen in the structure is bound to one silicon and three atoms in octahedral coordination.
Strictly speaking, olivines represent a solid-solution between forsterite (Fo: Mg2SiO4; named in honor of Adolarius Jacob Forster) and fayalite (Fa: Fe2SiO4; named after Fayal Island, Azores). Forsterite and fayalite form a continuous solid solution with Fe and Mg substituting each other in the M-sites. Fe2+ has a small preference for the M1 site. This solid solution is commonly indicated using the % of end members in the solution. For instance, an olivine with 30% forsterite and 70% fayalite is indicated as Fo30, Fa70, or Fo30Fa70. Similarly to the plagioclase series, different names for compositional intervals of olivine have been proposed (chrysolite, hyalosiderite, hortonolite, and ferrohortonolite; see figure), which did not receive the same success and are not commonly used.
Natural olivines show limited substitution of (Fe,Mg) for Ca. Ca is a very large cation and does not enter the structure of forsterite – fayalite easily. Ca-bearing olivines, monticellite (CaMgSiO4), kirschsteinite (FeMgSiO4), and larnite (Ca2SiO4), have larger unit cells compared to ferromagnesian olivines, allowing them to accommodate the Ca cation in their structure. Ca-bearing olivines are very rare and, in nature, occur as accessories in Ca-rich silica-undersaturated rocks (e.g. carbonatites) and metasomatic rocks and skarns deriving from impure limestones. Other cations that may be present within forsterite-fayalite are Mn, Cr, Ni, and Fe3+. Cr commonly occurs as thin lamellae of exsolved chromite within olivine.

Ternary diagram showing the minerals of the olivine group. Two main solid solutions occur: forsterite-fayalite and monticellite-kirschsteinite. Ca-olivines are rare in nature.


The gem variety of olivine: peridot. Note the squat prismatic habit. Size: 3.4 x 2.5 x 1.9 cm. Locality: Naran-Kagan Valley. Kohistan, Pakistan. Photo by Robert M. Lavinsky.

Properties
Habit: stocky prismatic-tabular, ‘barrel-shaped’ grains
Hardness: 6.5 – 7
Cleavage: {010}, {100} imperfect (2 perpendicular cleavages)
Twinning: {011}, {012}, {031}
Color: olive green, green, yellow-green, yellow-amber
Luster: vitreous
Streak: colorless
Alteration: serpentine, iddingsite, bowlingite, chlorophaeite
In thin section…
α(//b): 1.635 (forsterite) – 1.827 (fayalite)
β(//c): 1.651 (forsterite) – 1.869 (fayalite)
γ(//a): 1.670 (forsterite) – 1.879 (fayalite)
2Vγ: 82° (forsterite) – 134° (fayalite)
Color: colorless (forsterite) to pale yellow (fayalite)
Pleochroism: none (forsterite); α = γ pale yellow to β orange yellow, reddish brown (fayalite)
Birefringence (δ): 0.035-0.052 (high interference colors, higher in fayalite)
Relief: high
Optic sign: + or –
[Mindat]

Alteration
Olivine alters to fifty shades of alteration products, being very susceptible to hydrothermal alteration and weathering. During ocean floor metamorphism, olivine is commonly replaced by serpentine and brucite, as well as talc and various carbonates. Other common forms of alteration are iddingsite, bowlingite, and chlorophaeite.

Iddingsite: reddish brown replacement of olivine consisting of smectite, chlorite, goethite, and hematite.
Bowlingite: greenish alteration of olivine consisting of smectite, chlorite, serpentine, talc, white mica, and quartz.
Chlorophaeite: similar to iddingsite but more variable in color and containing less Fe3+.

Field features
Olivine in the field can be identified based on (1) its characteristic stocky habit with six-sided or eight-sided ‘barrel-like’ sections, (2) distinctive bottle green-yellowish transparent color, and (3) vitreous luster, similar to quartz. Olivine appears very different from pyroxene and amphibole, both showing well-developed cleavage planes (cleavage in olivine is poorly developed) and metallic luster. Alteration commonly destroys the beautiful green colors of olivine but makes the identification of olivine even easier: in mafic and ultramafic rocks, plagioclase and pyroxene often appear less altered than olivine, whereas olivine can be present only as pseudomorphs, partially or totally replaced by its alteration products, showing black to green to reddish colors.

Basalt with phenocrystals of slightly weathered olivine in a fine-grained groundmass. Most crystals are stocky and show 6- to 8-sided sections. Note the greenish color and vitreous luster. Oahu, Hawaii, USA. Width of sample 6 cm. Photo by Siim Sepp (sandatlas.org).

Weathered olivine is not olive green at all. Here, olivine crystals (recognizable for their characteristic shape) show dull, earthy colors, as fine-grained alteration products replace them. Black grains are pyroxene phenocrysts. Basanite from La Palma, Canary Islands, Spain. Photo by Siim Sepp (sandatlas.org).

Olivine in thin section
Mg-rich olivines (forsterite-rich), the commonest in nature, show high relief and are colorless at PPL. Euhedral grains show barrel-shaped six-sided or eight-sided sections. Olivine may also show other crystal habits, ranging from tabular to prismatic to acicular and form skeletal grains (e.g. spinifex olivine). Cleavage traces are imperfect and poorly-developed in comparison with pyroxene and amphibole: when present, they are oriented parallel to the long axis, intersecting at 90° on the equant, basal faces. At CPL, olivine shows high third order interference colors. Along the forsterite-fayalite series, refraction indices, birefringence, and relief increase with increasing content of Fe. Fayalite-rich olivine appears pale yellow and pleochroic (yellow to orange).
Alteration is common in olivine, which may appear as relic grains surrounded by serpentine or other alteration forms that appear reddish (iddingsite, chlorophaeite) or greenish (bowlingite) in thin section. Iddingsite may appear homogeneous and highly birefringent.
In skarns and metamorphosed limestones, several minerals may resemble olivine, for example epidote and humite.

CPL
CPL
CPL
PPL
PPL

Above: round olivine (high relief at PPL, high interference colors at CPL) and chromite crystals (opaque) surrounded by a calcite groundmass (high grey interference colors at CPL). Kimberlite from Bloemfontein, South Africa. Field of view: 7mm. Photo by Alessandro Da Mommio (alexstrekeisen.it).

Euhedral olivine crystal in a basalt. Note the pseudo-hexagonal six-sided shape and the high interference colors. An imperfect prismatic cleavage, oriented parallel to the long axis, is well visible. The sharp, thick and irregular surfaces are fractures. The surrounding groundmass contains abundant plagioclase crystals (with polysynthetic twinning, first-order grey). Etna Volcano, Italy. CPL Image. Field of view: 7mm. Photo by Alessandro Da Mommio (alexstrekeisen.it).

Olivine (high interference colors) and plagioclase (grey interference colors, with polysynthetic twinning). Olivine shows the typical ‘barrel’ shape. Basalt from Etna Volcano, Italy. CPL Image. Field of view: 7mm. Photo by Alessandro Da Mommio (alexstrekeisen.it).

CPL
CPL
CPL
PPL
PPL

Above: mesh texture: relics of olivine (high relief at PPL, high interference colors at CPL) in optical continuity surrounded by fibrous serpentine (transparent at PPL, grey at CPL). Field of view: 2mm. Photo by Alessandro Da Mommio (alexstrekeisen.it).

CPL
CPL
CPL
PPL
PPL

Above: iddingsite alteration (orange/reddish) on olivine rim. Field of view: 2 mm. Photo by Alessandro Da Mommio (alexstrekeisen.it).

CPL
CPL
CPL
PPL
PPL

Above: olivine crystal completely substituted by bowlingite (pseudomorphed) in a basalt from India. Bowlingite appears greenish at PPL and shows high interference colors at CPL. Field of view: 2 mm. Photo by Alessandro Da Mommio (alexstrekeisen.it).

Occurrence
Olivine is the main constituent of peridotites, where it commonly shows a forsterite-rich composition. Dunites are a subgroup of peridotites that consist almost entirely (> 90%) of olivine. Olivine is present also in a wide range of mafic and ultramafic igneous rocks, like cumulates, kimberlites, gabbros, basalts, and komatiites. Mg-rich olivine is incompatible with quartz and, in presence of enough silica, magmas produce pyroxenes. By contrast, Fe-rich olivine (fayalite) is stable in presence of quartz and may occur in Fe-rich alkaline granites, syenites and their volcanic counterparts.
In metamorphic rocks, olivine may form by (1) contact metamorphism or metasomatism of impure Fe-bearing limestone or sediment, and (2) high-temperature metamorphism of ultramafic rocks. In serpentinites, at high temperature, serpentine reacts with diopside or brucite to produce olivine. In impure marbles, olivine forms from the reaction between quartz or amphibole with dolomite. Fayalite may form in a similar fashion, from the reaction between quartz and Fe-rich carbonates (siderite, ankerite).

References and Further Reading
Buening, D. K., & Buseck, P. R. (1973). Fe‐Mg lattice diffusion in olivine. Journal of Geophysical Research78(29), 6852-6862.
Donaldson, C. H. (1976). An experimental investigation of olivine morphology. Contributions to mineralogy and Petrology57(2), 187-213.
Goetze, C. (1978). The mechanisms of creep in olivine. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences288(1350), 99-119.
Raleigh, C. B. (1968). Mechanisms of plastic deformation of olivine. Journal of Geophysical Research73(16), 5391-5406.
Roeder, P. L., & Emslie, R. (1970). Olivine-liquid equilibrium. Contributions to mineralogy and petrology29(4), 275-289.
Siever, R., & Woodford, N. (1979). Dissolution kinetics and the weathering of mafic minerals. Geochimica et Cosmochimica Acta43(5), 717-724.
Simkin, T., & Smith, J. V. (1970). Minor-element distribution in olivine. The Journal of Geology78(3), 304-325.

        

See also
Alexstrekeisen.it – Olivine
Sandatlas.org – Olivine

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Mineral Properties
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